Resistance and Ohm’s Law

When designing or using electronic devices, we often need to know how changing the voltage across the device will affect the current flowing through it.
For example, what will happen to the current flowing through a circuit element, such as a bulb that is part of a circuit, if we increase the voltage across it? Alternatively, what will happen to the current passing through a carbon resistor as voltage increases? To answer these questions, let’s start with two definitions: /

Definition 1:We define the resistance, R, of an electrical device as the ratio of the potential difference or voltage drop, V, across it to the current passing through it.

(definition of Resistance).

Definition 2: We say that a device obeys Ohm’s Law if the current passing through it is always directly proportional to the voltage drop across it, independent of the value of the voltage drop, so that the value of its resistance doesn’t change even if we apply a different voltage across it.

In this assignment you will compare the behavior of a flashlight bulb to that of a carbon resistor and determine whether or not these devices obey Ohm’s Law.

1. Preliminary Questions

Note: You will receive full credit for each prediction made in this preliminary section whether or not it matches conclusions you reach in the next section. As part of the learning process it is important to compare your predictions with your results. Do not change your predictions!

(a)  Which of the two meters is wired to measure the current passing through the bulb shown in Figure1? The one of the left or the one on the right? Note: The “light grey” wire in the bottom left in the photo is an enhancement of a red wire in the movie frame. If you still have trouble interpreting the wiring open the image entitled <Fig1_bulb_meter.pct> and use it instead of Figure 1.

Left Meter ______Right Meter ______

(b)  How many batteries are in the circuit in Figure 1?

Zero ______One ______Two ______Three ______

(c)  Draw a neat circuit diagram for Figure 1 using the following standard symbols:

(d)  Circle the only graph shown below that depicts a proportional relationship.

2. Activity-Based Questions

In this section, you will work with a Logger Pro file entitled OhmsLaw.cmbl in which you can enter meter readings shown in two movies. The first movie shows what happens to the current passing through a flashlight bulb when there are different voltage drops (or potential differences) across it when 0, 1, 2 and 3 batteries are hooked into the circuit. The second movie been created in a similar manner but the bulb has been replaced with a carbon resistor. Note: Before taking data you may want to play the movies and then enlarge them so you can see the numbers and the decimal points on the meters clearly! Take data for the voltage across and the current through the bulb: In <bulb_meter.mov use frames 0, 26 or 27, 43 or 44, and 64 or higher for recording data and enter values in the table below.

(a) Take data for the voltage across and the current through the resistor: In <resistor_meter.mov use frames 0, 28 or 29, 48 or 49, and 73 or higher and enter values in the table below.

i bulb
(mA) / V_bulb
(Volts) / i_resistor
(mA) / V_resistor (Volts)

(b) Graph your results: Sketch your results for the two devices in the graph frames below.

(c) Calculate the resistance for the flashlight bulb in ohms for each of your values of current and voltage drop (except i = 0 A and V = 0 V). Report your results to two significant figures. Hint:Don’t forget to change milliamps (mA) to amps (A).

R1 Bulb R2 Bulb R3 Bulb

(d) Does the flashlight bulb obey Ohm’s Law over the range of voltages used in the circuit? Why or why not? Cite evidence for your answer from both parts 2(b) and 2(c).

(e) Calculate the resistance for the carbon resistor in ohms for each of your values of current and voltage drop (except i = 0 A and V = 0 V). Report your results to two significant figures. Hint:Don’t forget to change milliamps (mA) to amps (A).

R1 Resistor R2 Resistor R3 Resistor

(f) Does the carbon resistor obey Ohm’s Law over the range of voltages used in the circuit? Why or why not? Cite evidence for your answer from both parts 2(b) and 2(e).

3. Reflections on Your Findings

(a)  If either or both of your circuit elements did not obey Ohm’s Law, can you think of any reasons for the non-ohmic behavior? In other words what might be happening to one or both of the elements as the applied voltage drop increases?

Physics with Video Analysis 28 - XXX